1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the construction of isolated MEMS devices using surface fabrication techniques.
2. Discussion of the Related Art
Microelectromechanical systems (MEMS) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS device is an accelerometer having a movable component that, in response to an external stimulus, is actuated so as to vary the size of a capacitive air gap. Accordingly, the capacitance output of the MEMS device provides an indication of the strength of the external stimulus.
One presently employed method of fabricating MEMS components uses bulk fabrication techniques employing a nonconductive substrate and a prefabricated wafer, such as a silicon-on-insulator (SOI) wafer. The wafer is bonded to the substrate, and is subsequently patterned to produce a MEMS device. Surface fabrication processes may then be used to deposit additional materials on the wafer if so desired. Additional processes are typically performed on the wafer because of the need to remove excess material on these wafers. This increases the amount of time needed to fabricate the MEMS device, and adds cost and complexity to the process. Furthermore, commercially available SOI wafers are generally expensive. SOI wafers are generally desirable when fabricating a MEMS device having sufficient thickness, on the order of 20 microns, which is difficult to attain using other known methods.
However, when fabricating a MEMS device having less thickness, it is desirable to avoid the use of expensive and limiting SOI wafers. Accordingly, a MEMS device may alternatively be constructed using exclusively surface fabrication processes. The aforementioned disadvantages associated with bulk fabrication are alleviated, since the desired materials are chosen and individually deposited to a desired thickness to fabricate the MEMS device. Furthermore, fabricating a MEMS device using surface fabrication techniques is generally less expensive than using commercially available SOI wafers.
Currently, when using surface fabrication techniques to fabricate a MEMS component, a sacrificial material, such as silicon dioxide, is deposited and patterned onto a substrate, such as single crystal silicon which has been covered with a layer of silicon nitride. A structural material, such as polysilicon, is deposited and patterned on top of the sacrificial material. Thus two materials are deposited onto the substrate to form the MEMS device. The structural material is etched to form a stationary conductive member and a movable MEMS element. The sacrificial material is then selectively etched to release the movable MEMS element from the substrate and the stationary conductive member, thereby rendering the MEMS device operational. This leaves only a single material, the structural material.
One significant disadvantage associated with current surface fabrication techniques involves the lack of electrical isolation that is achieved. The present inventors have discovered that MEMS devices may be used as a current or voltage sensor, in which the device may receive high voltages at one end of the device, and output an electrical signal at the other end of the device to, for example, a sensor. The output could be a function of the capacitance of the MEMS device, as determined by the position of the movable MEMS element with respect to the stationary element. However, because the entire movable MEMS element achieved using conventional surface fabrication techniques is conductive, the input and output ends of the MEMS device are not sufficiently isolated from one another, thereby jeopardizing those elements disposed downstream of the MEMS output.
What is therefore needed is a method for fabricating a MEMS device using surface fabrication techniques that provides sufficient electrical isolation for the device.
The present inventors have recognized that the addition of an insulating layer to portions of the movable MEMS element of a MEMS device constructed in accordance with surface fabrication techniques provides adequate electrical isolation, thereby allowing the MEMS device to be operable in a wide range of applications.
In accordance with one aspect of the invention, a method is provided for constructing a MEMS device having a first stationary conductive member separated from a second movable conductive member by a variable size gap. The method uses exclusively surface fabrication techniques, and begins by providing a substrate, and depositing a sacrificial material onto the substrate to form a sacrificial layer. An insulating material is deposited onto the sacrificial layer to form an insulating layer. Next, a conductive material is deposited onto the insulating layer to form a conductive layer. A portion of the conductive layer is then etched through to the insulating layer to form the first and second adjacent conductive structures separated by a variable size gap. A portion of the insulating layer is then etched to provide a base for the second conductive structure. Finally, a portion of the sacrificial layer is etched to release the base and second conductive structure from the substrate.
In accordance with another aspect of the invention, a wafer level cap is attached to the fabricated MEMS device.
In accordance with another aspect of the invention, electrical traces are formed within the device that enables electrical communication with the ambient environment.
These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must be made therefore to the claims for this purpose.